<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        .box {
            position: relative;
            height: 2.5rem;
            /* background-color: green; */
            /* margin-bottom: 10px; */
        }
        .box::after {
            content: '';
            position: absolute;
            bottom: 0;
            left: 0;
            right: 0;
            /* 物理像素
            css 像素 */
            height: 1px;
            background-color: black;
            transform: scaleY(50%);
            transform-origin: 0 100%;
        }
    </style>
<body>
    <div class="box"></div>
    <div class="box"></div>
</body>
</html>